Heretofore, an optical system commonly known as the Schlieren system has been utilized to study density gradients in transparent substances by viewing refractive index gradients. In the Schlieren method, light from a slit is collimated from a lens and focused onto a knife edge by a second lens. The text volume being studied is placed between the two lenses and the deflection pattern that results is viewed on a screen placed behind the knife edge. The deflection is related to the refractive index gradients which is, in turn, related to the density gradient in the test volume. A more complete discussion of Schlieren apparatus may be found in Liepmann and Roshko, Elements of Gas Dynamics, Wiley & Sons, Inc., New York, 1957. However, the conventional system is capable of visualizing only one component of the index gradient at a time.
It is advantageous to be able to observe two perpendicular components simultaneously in a dynamic situation so that both components can be correlated. Without such correlation, it is impossible to tell which physical features are causing which phenomena. For example, in a moving fluid, information would be lost when adjustment is made from one knife edge position showing one gradient component to another knife edge position showing a different gradient component. The image changes during this time period, and this would be true in any dynamic situation wherein a test volume is changing its refractice index in some way. In such a situation, it is impossible to track one particular part of the fluid flow image and to correlate it with the other image showing a different concentration gradient component. Either the density is changing, fluid motion is occurring, or some changes are being made in the specimen during the observation period; the material specimen is being stressed so that it is changing.